A fuel cell is intended to cause an electrochemical reaction between a fuel such as a hydrogen gas and an oxidant such as air (oxygen gas) so as to convert a chemical energy of the fuel directly into an electric energy. Among various fuel cells, a solid polymer electrolyte fuel cell has excellent characteristics such as high power density, structural simplicity, and relatively low operating temperature, and thereby its technical development has been increasingly driven forward.
FIG. 21 shows one example of conventional solid polymer electrolyte fuel cells to be used on the ground. This solid polymer electrolyte fuel cell comprises a fuel cell stack including a solid polymer electrolyte membrane 1, fuel and oxidant electrodes 2, 3 disposed to sandwiching the solid polymer electrolyte membrane 1 therebetween, a hydrogen supply port 5 provided on the upper side of the fuel electrode 2, and an air (oxygen gas) supply port 6 provided on the upper side of the oxidant electrode 3. In the solid polymer electrolyte fuel cell, a moisture or water is essential to allow hydrogen ions to be moved in the solid polymer electrolyte membrane 1. For this reason, humidifiers 7, 8 are connected, respectively, to the hydrogen supply port 5 and the air supply port 6 in order to prevent the drying out of the solid polymer electrolyte membrane 1. The fuel cell further includes a hydrogen discharge port 9 provided on the lower side of the fuel electrode 2, and an air (oxygen gas) discharge port 10 provided on the lower side of the oxidant electrode 3.
A hydrogen gas obtained by reforming a hydrocarbon-based fuel such as methanol, gasoline or methane gas is introduced into the humidifier 7 on the side of the fuel electrode 2, and an air pressurized up to a predetermined pressure by a compressor (not shown) is introduced into the humidifier 8 on the side of the oxidant electrode 3. The hydrogen gas and the air are humidified by humidifiers 7, 8, and then supplied to the sides of the fuel electrode 2 and the oxidant electrode 3 through the hydrogen supply port 5 and the air supply port 6, respectively. The hydrogen gas and the air supplied to the sides of the fuel electrode 2 and the oxidant electrode 3 cause an electrochemical reaction therebetween to generate an electric power while flowing in parallel with one another along the solid polymer electrolyte membrane 1, and then discharged out of the fuel cell stack 4 through the hydrogen discharge port 9 and the air discharge port 10, respectively.
There is another example of conventional solid polymer electrolyte fuel cells, which was used for Gemini in the 1960s. This solid polymer electrolyte fuel cell uses styrene-based solid polymer as the electrolyte, a pure hydrogen gas as the fuel, and a pure oxygen gas as the oxidant. In this case, an absorbent wick is provided adjacent to the electrodes to absorb and remove any water created in the fuel cell stack during the reaction between the pure hydrogen and the pure oxygen, and to naturally evaporate the absorbed water so as to humidify the interior of the fuel cell stack.
As announced from the American Institute of Aeronautics and Astronautics (AIAA) in 1999, a solid polymer electrolyte fuel cell is employed in the ULDB (Ultra Long Durability Balloon) as a NASA's aeronautical plan for the Stratosphere Platform Project. While this fuel cell also uses a pure hydrogen gas as the fuel, and a pure oxygen gas as the oxidant, any humidifier is omitted to simplify system, and the fuel cell stack is disposed with a slope to allow any water created in the fuel cell stack to be dropped out of the fuel cell stack without recovering.
The former conventional example as shown in FIG. 21 uses air as an oxygen gas source. Thus, if an unreacted gas of the air is re-circulated in the fuel cell stack 4, a nitrogen gas as an inert gas in the air will be accumulated in the fuel cell stack 4, which leads to lowered partial pressure of oxygen and significantly deteriorated fuel-cell characteristics. From this cause, it has been difficult to re-circulate any unreacted gas in the fuel cell stack 4 and provide enhanced oxidant utilization efficiency. Further, it has also been difficult to re-circulate an unreacted gas of the fuel in the fuel cell stack 4, because the re-circulation of the unreacted gas causes the accumulation of CO2 and/or unreformed fuel in fuel cell stack 4, resulting in deteriorated fuel-cell characteristics.
In addition, when air is used as the gas on the side of the oxidant electrode 3, the content of oxygen in the air is about 21%, and consequently about 79% of nitrogen will be supplied to the fuel cell. Thus, the nitrogen as an inert gas is circulated in the fuel cell stack 4 all the time, and the ratio of water vapor is increased up to only 34.7% at 100% of oxygen utilization due to the presence of the nitrogen. When the fuel cell is operated at 50% of utilization ratio of oxygen in the air under normal pressures, it is required to control its operating temperature equal to or less than 59° C., and thereby the allowable range of the operating temperature will be undesirably narrowed.
In consequence of inevitably discharging the air outside, the water created in the fuel cell stack 4 is discharged out of the fuel cell stack 4 together with the air. Thus, the solid polymer electrolyte membrane, particularly the vicinity of the air supply port is apt to dry and have an undesirably reduced region for contributing the reaction. For this reason, it has been required to provide the humidifier 7, 8 in both or either one of the hydrogen supply port 5 and the air supply port 6, resulting in difficulties in downsizing and/or weight reduction of the fuel cell.
On the other hand, the latter conventional example in the ULDB is adapted to allow the water created in the fuel cell stack to be dropped out of the fuel cell stack by. gravitation. Thus, the created water is removed dependent on the dropping speed, resulting in low applicability to high power generation. In addition, the interior of the fuel cell stack is humidified by the natural evaporation of the water absorbed in the wick, without any control of the humidity in the fuel cell stack. Besides, the removed water is directly discharged outside without recovering.